Fix before_prompt event on old GDB versions (#464)

* Fix before_prompt event on old GDB versions

This adds an `EventWrapper` class which behaves similar to gdb events but lets us:
* check whether event is a real gdb event or not
* call event callbacks if it is not a real gdb event

* Better comment

pwndbg/events.py

@@ -39,12 +39,15 @@ class StartEvent(object):
     def __init__(self):
         self.registered = list()
         self.running    = False
+
     def connect(self, function):
         if function not in self.registered:
             self.registered.append(function)
+
     def disconnect(self, function):
         if function in self.registered:
             self.registered.remove(function)
+
     def on_new_objfile(self):
         if self.running or not gdb.selected_thread():
             return
@@ -62,8 +65,46 @@ class StartEvent(object):
     def on_stop(self):
         self.on_new_objfile()
 
+
 gdb.events.start = StartEvent()
 
+
+class EventWrapper(object):
+    """
+    Wraper for GDB events which may not exist on older GDB versions but we still can
+    fire them manually (to invoke them you have to call `invoke_callbacks`).
+    """
+    def __init__(self, name):
+        self.name = name
+
+        self._event = getattr(gdb.events, self.name, None)
+        self._is_real_event = self._event is not None
+
+    def connect(self, func):
+        if self._event is not None:
+            self._event.connect(func)
+
+    def disconnect(self, func):
+        if self._event is not None:
+            self._event.disconnect(func)
+
+    @property
+    def is_real_event(self):
+        return self._is_real_event
+
+    def invoke_callbacks(self):
+        """
+        As an optimization please don't call this if your GDB has this event (check `.is_real_event`).
+        """
+        for f in registered[self]:
+            f()
+
+
+# Old GDBs doesn't have gdb.events.before_prompt, so we will emulate it using gdb.prompt_hook
+before_prompt_event = EventWrapper('before_prompt')
+gdb.events.before_prompt = before_prompt_event
+
+
 # In order to support reloading, we must be able to re-fire
 # all 'objfile' and 'stop' events.
 registered = {
@@ -72,7 +113,7 @@ registered = {
     gdb.events.new_objfile: [],
     gdb.events.stop: [],
     gdb.events.start: [],
-    gdb.events.before_prompt: []
+    gdb.events.before_prompt: []  # The real event might not exist, but we wrap it
 }
 
 # GDB 7.9 and above only
@@ -82,20 +123,24 @@ try:
 except (NameError, AttributeError):
     pass
 
+
 class Pause(object):
     def __enter__(self, *a, **kw):
         global pause
         pause += 1
+
     def __exit__(self, *a, **kw):
         global pause
         pause -= 1
 
+
 # When performing remote debugging, gdbserver is very noisy about which
 # objects are loaded.  This greatly slows down the debugging session.
 # In order to combat this, we keep track of which objfiles have been loaded
 # this session, and only emit objfile events for each *new* file.
 objfile_cache = set()
 
+
 def connect(func, event_handler, name=''):
     if debug:
         print("Connecting", func.__name__, event_handler)
@@ -130,25 +175,31 @@ def connect(func, event_handler, name=''):
     event_handler.connect(caller)
     return func
 
+
 def exit(func):        return connect(func, gdb.events.exited, 'exit')
 def cont(func):        return connect(func, gdb.events.cont, 'cont')
 def new_objfile(func): return connect(func, gdb.events.new_objfile, 'obj')
 def stop(func):        return connect(func, gdb.events.stop, 'stop')
 def start(func):       return connect(func, gdb.events.start, 'start')
 
+
 before_prompt = partial(connect, event_handler=gdb.events.before_prompt, name='before_prompt')
 
+
 def reg_changed(func):
     try:
         return connect(func, gdb.events.register_changed, 'reg_changed')
-    except Exception:
+    except AttributeError:
         return func
+
+
 def mem_changed(func):
     try:
         return connect(func, gdb.events.memory_changed, 'mem_changed')
-    except Exception:
+    except AttributeError:
         return func
 
+
 def log_objfiles(ofile=None):
     if not (debug and ofile):
         return
@@ -158,8 +209,10 @@ def log_objfiles(ofile=None):
     print("objfile: %r" % name)
     gdb.execute('info sharedlibrary')
 
+
 gdb.events.new_objfile.connect(log_objfiles)
 
+
 def after_reload(start=True):
     if gdb.selected_inferior().pid:
         for f in registered[gdb.events.stop]:
@@ -168,6 +221,9 @@ def after_reload(start=True):
             if start: f()
         for f in registered[gdb.events.new_objfile]:
             f()
+        for f in registered[gdb.events.before_prompt]:
+            f()
+
 
 def on_reload():
     for event, functions in registered.items():
@@ -175,18 +231,22 @@ def on_reload():
             event.disconnect(function)
         registered[event] = []
 
+
 @new_objfile
 def _start_newobjfile():
     gdb.events.start.on_new_objfile()
 
+
 @exit
 def _start_exit():
     gdb.events.start.on_exited()
 
+
 @stop
 def _start_stop():
     gdb.events.start.on_stop()
 
+
 @exit
 def _reset_objfiles():
     global objfile_cache

pwndbg/prompt.py

@@ -58,4 +58,13 @@ def set_prompt():
     gdb.execute('set prompt %s' % prompt)
 
 
-gdb.prompt_hook = prompt_hook
+if pwndbg.events.before_prompt_event.is_real_event:
+    gdb.prompt_hook = prompt_hook
+
+else:
+    # Old GDBs doesn't have gdb.events.before_prompt, so we will emulate it using gdb.prompt_hook
+    def extended_prompt_hook(*a):
+        pwndbg.events.before_prompt_event.invoke_callbacks()
+        return prompt_hook(*a)
+
+    gdb.prompt_hook = extended_prompt_hook